As a young lad who liked to shoot monsters in the eyes first and ask questions later, for a good while I kind of just assumed that in Fallout's universe the nuclear bombs dropped around the 1960s, cementing the series' signature retro-futuristic aesthetic. Now I know this is not the case and you probably know it as well. But if you'd like to know exactly what happened and when in that particular world, you should check out this PC Gamer article that offers a chronological list of Fallout's major events. An excerpt:

Below is a timeline of the major events in Fallout's alternate history as it exists right now. While there have been references to events going back as far as the 17th century in the games (like the alien abductions in Fallout 3's Mothership Zeta expansion), the broad sweep of history in the Fallout universe resembles ours right up to the end of World War 2. It's only after 1947, when transistors were popularized in our world but not that of Fallout, that divergence begins, and only in the 1960s that those differences becomes significant.

1961 Captain Carl Bell of the United States Space Administration, piloting the Defiance 7, becomes the first becomes human in space. According to the Museum of Technology in Fallout 3, "this has been constantly refuted by both the Soviet Union and China."

1969 The USA is divided into the Thirteen Commonwealths, each comprising several of the existing states, and changes its flag. The Valiant 11 lunar lander touches down and Captains Richard Wade, Mark Garris, and Michael Hagen of the USSA become the first astronauts to walk on the surface of the moon. In 308 years time, the Valiant 11's dish will be repurposed to increase the distance over which Three Dog can play 'Butcher Pete' at people.

1992 The Sierra Army Depot changes its mission statement, becoming "Responsible for the demilitarization of stores of surplus ammunition". Eventually, it will become home to the Skynet AI as seen in Fallout 2.

2020 The USSA commissions the creation of the Delta IX, the final model of manned moon rocket. Eventually the Delta IX rockets will be repurposed, their crew and instruments replaced by nuclear warheads.